One of the main applications of the invention is mounting burners in ducts that exhaust postcombustion gases downstream from gas turbines and upstream from recuperator boilers in a cogeneration unit: such burners enable the temperature of turbine gases to be raised up to 1800.degree. C., for example, thereby increasing the efficiency of said boilers. In addition, they make it possible to operate said boilers with fresh oxidizing air when the turbines are not in operation.
Other applications are also possible for heating gas ducts, in particular those having a low oxygen content, such as installations for heating flue gases. . . .
At present there exist four types of burner corresponding to the above applications; i.e. the following, listed by type of structure:
strip burners which use only gas as the sole fuel and which are disposed in fixed manner inside said duct for the gas to be heated; PA1 gas and light fuel oil strip burners comprising fixed gas burner strips as before disposed between burner heads for light fuel oil, such as those described in patent application FR 2 285 574; together, the strips and the heads constitute a single assembly and they do not enable heavy fuel oil to be used since it cannot be distributed uniformly between the various heads, since it is practically impossible to balance the heavy fuel oil flow rate accurately without metering because of the risk of solidification, because of the differences of headloss as a function of feed line length and because of the differences in burner height; that is why heavy fuel oil burners generally make use of a single central injection pipe; PA1 lateral or external burners having a flame axis that is perpendicular or inclined relative to the flow direction of the gas to be heated, i.e. to the main flow axis of the duct. This disposition makes it possible to place the burner(s) outside the duct, providing an intersecting axis flame is given protection, or to place it in a housing that is completely outside the duct, in which case the burner does not heat the gas directly but acts as a hot gas generator, or by placing the flame inside the duct and protecting it by a kind of internal deflector to keep the gases that are to be heated away from the transverse flame so as to prevent the flame being excessively disturbed and so as to avoid producing too large a quantity of unburned fuel. Nevertheless, that disposition suffers from numerous drawbacks, in particular: it is difficult to match the size of the flame or flames exactly to the size of the duct; the recuperation boiler situated downstream from the burner(s) does not recover radiation from the flames, thus giving rise to a loss of efficiency and requiring overdimensioning; the transverse flames are always disturbed to some extent by the flow of gases to be heated which have a low oxygen content thus giving rise to unburned fuel; and the sides of the duct can be damaged by the proximity of flame ends; and PA1 burners that can be referred to as "in-duct" burners since they are located entirely inside a gas flow duct; this is the type of installation used for the burners of the present invention, and it has a burner flow axis parallel to the flow axis inside the duct; the number of burners installed in this way is small, and is generally always equal to or less than six, and is even preferably two; the burners are fed with liquid or gaseous fuels by pipes connected to the side of the duct. Compared with the burners in the first two types of structure mentioned above, these burners provide far more options since they can use any type of fuel equally well, regardless of whether the fuel is liquid, light, heavy, or gaseous, and secondly they give combustion results that are better than those of the third above-described structure because the burners are positioned much more logically on the axis and near the center of the duct; however, unlike lateral or external burners, but like the burners in the first two structures described, they are very difficult of access, and in order to dismount them, modify them, or repair them it is necessary to enter into the duct. This is not too inconvenient for gas-only strip burners or even for light fuel oil burners since the risks of having to perform adjustments and maintenance operations on such strips are much lower than when using heavy fuel oil. However, the use of special gases, such as hydrogenated gases, or gases that entrain liquid particles such as refinery gases, makes it necessary to be able to have access likewise for dismounting and maintenance of gas injectors.
Unfortunately, the ducts in which the gases that are to be heated or reheated flow are several meters wide and tall, commonly about 6 meters (m), and given that turbine exhaust gases are already at about 500.degree. C. or more, in order to be able to act on such burner heads it is necessary to stop the flow of hot air and wait for the equipment to cool down before it is possible to penetrate inside the duct to access said burners.